You are just drawing different conclusions from some of his experiences. He drew his own personal conclusions. Its all a matter of perspective. Like I said, people think differently. And his way of thinking, considering how his life had been, isn't illogical at all. You just don't agree with the conclusion he reached because you are focusing and giving more importance to certain aspects of what he experienced, while he is focusing and giving more importance to others.
(Btw, you don't know if most were grateful or not. It didn't show much of that in the flashback, you are just assuming that most of the people Midora saw acted mostly as the ones Toriko saw, which isn't implied at all in the chapter. If anything is implied, its the opposite. As far as you know, its perfectly possible that, since the world was in eternal war, people were hopeless, even when someone gave them food, and many acted rude due to their hopelessness.)
And I don't think we are supposed to feel very sympathetic towards Midora… What the flashback was implying is that he had a rough life, drew the wrong conclusions, and lost himself in a self imposed pit of anger and sadness that gave born to an extreme eating compulsion. He is neither just a poor understood child or a true and complete monster.
If Frohze didn't die, he had no reason to lose himself in his own pit of anger and sadness and develop that extreme eating compulsion. I don't believe that he would ever truly share Frohze's opinion and vision (unless something else happened), and so he probably wouldn't really want to carry Acacia's mission like his brothers, but just because he disliked humanity and whatnot, it doesn't mean that he would do what he decided to do after Frohze died. So, he wouldn't be evil, or at least nowhere near how evil he is now. He probably would just prefer to live away from people, minding his own things.